Hi Brian,
The vehicles_land.bgl replacement fixes a corrupt file that is in all of the P3Dv2,3 & 4.X distributions. Without the replacement, road vehicles sometimes show with no tires. So, you should keep the replacement. However, that replacement file does not cause vehicle traffic to appear.
The density of all traffic is controlled in the P3D options section under traffic. I have my road vehicles slider set to 20% and I do see traffic along the shore roads across the channel from the PAKT airport. It is not heavy but it is there. I an using ORBX PAKT in P3Dv4.3.
Rod